Page 49 - Low Temperature Energy Systems with Applications of Renewable Energy

Principles and operation of refrigeration and heat pump systems 37

three-way valve and circulating pump that would supply water into the coil with the temper-

ature of 35 C should be installed in the “warm floor” system coil as well.
• Heating by means of radiators
We recommend choosing the temperature difference taking into account a maximum heat
carrier temperature of 45/55 C for such a system. When designing one should calculate

nominal readings of heating device power and increase their area by decreasing the temper-
ature gradient from 90/70 C to 55/45 C. In short, the power of one rib at the temperature

difference of 55/45 C is approximately 40% as compared with the power at temperature of
90/70 C. The heat pump can be directly connected to the heating system for a simple single-

loop system. If the system has two or more heating loops, a circulating pump should be
installed for each one. Precise calculation is an important factor, i.e., an additional thermo-
electric heater should consume as little energy as possible.
1.7.3 Monovalent and bivalent operating modes

Heat pumps can operate in various operating modes depending on the heat source that
is used by the heat pump, as well as on the designed building heating unit or heating
technique employed in the building. Four modes are discussed below.
• Monovalent mode
At this mode the heat pump covers the total heat demand for heating and hot water supply
(HWS). Soil and ground waters are the best heat sources since they are almost independent of
ambient temperature and give quite enough heat even at low ambient temperatures. The
monovalent mode is recommended for operating drain water heat pumps.
• Monopower engineering mode
To cover power loading peaks, heat pump units (HPUs) operating in the monopower
mode are provided with an additional electric heater that can support heating and, if possible,
hot water supply. In this case the additional electric heater also allows increasing the temper-
ature of additional hot water from time to time to prevent bacterium (legionella) formation.
When designing the HPU with integrated additional heating of hot water between the heat
pump and regulating devices, it is advisable to provide a tank-accumulator and, if necessary,
to equip it with a heating device. The tank accumulator incorporates several functions: (a)
short-time accumulation and tariff compensation for electric energy; (b) a bivalency switch;
and (c) a thermohydraulic regulating device. If the tank is made with an internal float flow
dish for heating the hot water, then it can be used for heating such water as well.
One more advantage of this variant is that the water temperature in the tank is changed
depending on ambient air temperature. Such a regulating model provides the balance of
heat pump operation in the maximum profitable mode, since the heat coefficient depends
on the rate of decreasing water temperature in radiators. In other cases the calculations of
the heating system do not differ from other classical systems. The bivalent heat pump plant,
which incorporates a second (back-up) heat generator, operates using the same kind of energy
(electric power). In the heating mode, the heat pump plant is supplemented by a heat generator
operating from electricity. The heat pump plant is selected from the calculation of 70e85% of
the maximum heat consumption of the building in accordance with DIN EN 12831.
• Bivalent and parallel mode
Heating units with a bivalent and parallel operation mode are equipped with both a heat
pump and an additional heat generator. For example, a liquid-full boiler was often used side
by side with an air-water heat pump in some apartment buildings a few years ago, with the